This is a repository copy of Mineralogy of sulfides. White Rose Research Online URL for this paper: http://eprints.whiterose.ac.uk/113362/ Version: Accepted Version Article: Vaughan, D.J. and Corkhill, C.L. orcid.org/0000-0002-7488-3219 (2017) Mineralogy of sulfides. Elements , 13 (2). pp. 81-87. ISSN 1811-5209 https://doi.org/10.2113/gselements.13.2.81 [email protected]https://eprints.whiterose.ac.uk/ Reuse Unless indicated otherwise, fulltext items are protected by copyright with all rights reserved. The copyright exception in section 29 of the Copyright, Designs and Patents Act 1988 allows the making of a single copy solely for the purpose of non-commercial research or private study within the limits of fair dealing. The publisher or other rights-holder may allow further reproduction and re-use of this version - refer to the White Rose Research Online record for this item. Where records identify the publisher as the copyright holder, users can verify any specific terms of use on the publisher’s website. Takedown If you consider content in White Rose Research Online to be in breach of UK law, please notify us by emailing [email protected] including the URL of the record and the reason for the withdrawal request.
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This is a repository copy of Mineralogy of sulfides.
White Rose Research Online URL for this paper:http://eprints.whiterose.ac.uk/113362/
Version: Accepted Version
Article:
Vaughan, D.J. and Corkhill, C.L. orcid.org/0000-0002-7488-3219 (2017) Mineralogy of sulfides. Elements , 13 (2). pp. 81-87. ISSN 1811-5209
Unless indicated otherwise, fulltext items are protected by copyright with all rights reserved. The copyright exception in section 29 of the Copyright, Designs and Patents Act 1988 allows the making of a single copy solely for the purpose of non-commercial research or private study within the limits of fair dealing. The publisher or other rights-holder may allow further reproduction and re-use of this version - refer to the White Rose Research Online record for this item. Where records identify the publisher as the copyright holder, users can verify any specific terms of use on the publisher’s website.
Takedown
If you consider content in White Rose Research Online to be in breach of UK law, please notify us by emailing [email protected] including the URL of the record and the reason for the withdrawal request.
Article for Elements Magazine on ‘Sulphides’ (Kiseeva and Edmonds, editors)
Mineralogy of Sulphides
David J. Vaughan1 and Claire Corkhill2
1 School of Earth, Atmospheric and Environmental Sciences, and Williamson Research Centre for Molecular Environmental Science, University of Manchester, Manchester M13 9PL, UK 2 NucleUS Immobilisation Science Laboratory, Department of Materials Science and Engineering, University of Sheffield, Sheffield S1 3JD, UK
Metal sulphides are the most important group of ore minerals. As shown in this brief introduction,
much is known about their compositions, crystal structures, phase relations and paragenesis. Much
less is known about their surface chemistry and, in particular, about their biogeochemistry, and about
the formation and behaviour of ‘nanoparticle’ sulphides, whether formed abiotically or biogenically.
These are large and complex topics which can only be touched upon in this article which also serves
to direct readers to more comprehensive accounts.
KEYWORDS: metal sulphides, crystal structures, paragenesis, surface chemistry, biogeochemistry
INTRODUCTION
Sulphide minerals are compounds in which sulphur is combined as an anion with a metal (or semi-
metal) cation or cations. The definition is commonly widened to include minerals in which the anion
is As or Sb, sometimes together with S, and to include Se and Te minerals. The sulphosalts are a
special subset of the sulphide minerals that have a general formula AmTnXp and in which the
common elements are A = Ag, Cu, Pb; T = As, Sb, Bi; X = S. They generally contain pyramidal TS3
groups in their structures. Although several hundred sulphide minerals are known, only five are
sufficiently abundant as accessory minerals in rocks to have been categorized as ‘rock forming’ (see
Bowles et al. 2011). They are pyrite, pyrrhotite, galena, sphalerite and chalcopyrite. Of these, it is the
iron sulphides (pyrite and pyrrhotite) which are dominant. The very fine particle iron sulphides found
in reducing environments beneath the surfaces of some sediments and soils are also volumetrically
important. Formally known as ‘amorphous iron sulphides’, they are now known to be comprised of
fine particle mackinawite (tetragonal FeS) and, to a lesser extent, the thiospinel mineral greigite
(Fe3S4), and to be metastable compared to pyrite and pyrrhotite.
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Above all, however, the sulphides are the most important group of ore minerals, responsible for the
concentration of a wide range of metals as mineable ore deposits. They are also potential sources of
pollution, of the air and of surface waters and soils. Pollution of the air may arise not only from the
smelting of sulphide ores, but also from the burning of coal which contains sulphur mainly as
sulphide impurities. The breakdown of sulphides exposed by weathering at the Earth’s surface
generates sulphuric acid as well as releasing potentially toxic metals into waters and soils. This very
common form of pollution may arise from minewastes (and termed acid mine drainage) or sulphide-
containing natural rocks (acid rock drainage).
The literature on sulphide minerals is extensive, and a number of overview textbooks and
monographs have been published; earlier work is reviewed in Ribbe (1974), Vaughan and Craig
(1978) and Kostov and Mincheeva-Stephanova (1981). The most recent comprehensive review is
that of Vaughan (2006). The present article provides a brief overview of the compositions and crystal
structures of the major sulphide minerals, aspects of their chemistries (bulk and surface) and their
occurrence. In addition to the sulphides mentioned above, pentlandite ((Fe,Ni)9S8) and its alteration
product violarite (FeNi2S4) are important as the major ore minerals of nickel, bornite (Cu5FeS4) as a
major copper mineral, and molybdenite (MoS2) as the source of world supplies of molybdenum.
Tetrahedrite (Cu12Sb4S13) is notable because of the large range of metals, silver in particular, which
can substitute at percent levels for copper or antimony in its structure. In contrast, arsenopyrite
(FeAsS) is the major natural source of arsenic, an extremely toxic pollutant.
CRYSTAL STRUCTURES AND THE CLASSIFICATION OF SULPHIDES
The structures of nearly all mineral sulphides are described by Makovicky (2006), who also provides
a detailed account of structural classification schemes. This extensive review provides much more
detail than we require here, where we categorize the most important sulphides into a series of groups
based on major structure types or having key structural features in common, as shown in Table 1
(modified after Vaughan and Craig, 1978). Commonly, these are the structures exhibited by a much
larger group of crystalline solids, such as the rocksalt structure of the galena group (Fig. 1a), the
sphalerite and wurtzite forms of ZnS (Fig. 1b,c), or the nickel arsenide structure (Fig. 1d).
The disulphides contain dianion units (S-S, S-As, As-As etc); here, in the pyrite structure, FeS6
octahedra share corners along the c-axis direction, whereas in the marcasite form of FeS2, the
3
octahedra share edges to form chains of octahedra along the c-axis. Loellingite (FeAs2) and
arsenopyrite (FeAsS) have variants of the marcasite structure with, respectively, shorter or
alternately long and short metal-metal distances across the shared octahedral edge.
Sulphides such as covellite (CuS; Fig. 1f) and molybdenite (MoS2) have layer structures, and a
number have structures best described as having rings or chains of linked atoms (such as realgar,
AsS). A diverse group, have been defined as the metal-excess group by Vaughan and Craig (1978);
they have metal:sulphur ratios greater than 1:1 and structures of the type well illustrated by
pentlandite ((Ni,Fe)9S8), the major ore mineral of nickel (see Fig. 1g).
In many of these groups, as well as several minerals exhibiting the actual structure type (see Table
1), there are other minerals that have structures based upon these ‘parent’ structures and that can be
thought of as being ‘derived’ from them. The relationship between derivatives and parents can
involve:
(1) distortion as in the troilite form of FeS which is simply a distortion of the parent NiAs
structure (Fig. 2a);
(2) ordered omission as in monoclinic pyrrhotite (Fe7S8) which is derived from the NiAs
structured FeS by removal of Fe atoms leaving holes (vacancies) that are ordered (see Fig.
2a);
(3) ordered substitution as in the structure of chalcopyrite (CuFeS2) which is derived from that
of sphalerite (ZnS) by the alternate replacement of Zn atoms by Cu and Fe resulting in an
enlarged (tetragonal) unit cell (see Fig 2b). Stannite (Cu2FeSnS4) results from further ordered
substitution of half of the Fe atoms in chalcopyrite by Sn (Fig. 2b);
(4) a stuffed derivative (Fig. 2c) as in talnakhite (Cu9Fe8S16), mooihoekite (Cu9Fe9S16) and
haycockite (Cu4Fe5S8) which are derived from chalcopyrite by the occupation of additional,
normally empty, cavities in the structure.
The structural relationships involved may be more complex in other cases. For example, some
sulphosalts are comprised of slabs or other units of the parent structure.
CHEMISTRY OF SULPHIDES
Bulk Chemistry
The chemical compositions of the sulphide minerals have been well characterized as a result of
numerous analyses of natural samples and laboratory investigations of phase equilibria (see Table 1
4
for names and formulae of all common, and many less common, sulphides). Although most sulphide
minerals can ideally be regarded as simple binary or ternary compounds, natural sulphides always
contain impurities ranging from trace (ppm) to minor (<5 wt %) amounts. Such impurities may
include toxic elements such as arsenic, cadmium and mercury. The more extensive substitutions
associated with solid solution are also found in the sulphides; for example, the complete solid
solution between pyrite (FeS2) and vaesite (NiS2) to give the intermediate composition mineral
bravoite ((Fe,Ni)S2 which can also have a cattierite (CoS2) component.
Certain metal sulphides also exhibit nonstoichiometry and, in some cases, the deviation of the
formula from a simple ratio is considerable. For example, pyrrhotite is commonly given the general
formula Fe1−xS where 0 < x < 0.125; here, the varying compositions correspond to varying
concentrations of vacancies in sites that would otherwise be occupied by iron atoms. However, in
systems such as these, ordering of the vacancies occurs at low temperatures, and the result may be a
series of stoichiometric phases of slightly different compositions. Although Fe7S8 has a (monoclinic)
superstructure resulting from vacancy ordering (see Fig. 2a), the situation in the so-called
‘intermediate’ or ‘hexagonal’ pyrrhotites is more complex. Some of these pyrrhotites may represent
ordered phases with clearly defined compositions (Fe9S10, Fe11S12 etc.), but more complex and
partial ordering in these systems may occur. One problem is that free-energy differences between a
series of phases resulting from vacancy ordering are very small, making a successful investigation of
the relationships between synthetic products very difficult. Further examples of nonstoichiometry in
sulphides may involve only small deviations from the simple ratio. For example, galena (PbS)
exhibits a range of nonstoichiometry of 0.1 atomic%. Apparently, galena is stable over a wide range
of values of aS2 (activity of sulphur), and at high aS2 it has lead vacancies, whereas at low aS2 there
are sulphur vacancies; the presence of Pb-vacancies in galena has been reported to confer enhanced
oxygen adsorption, which has important implications for the oxidative dissolution of this mineral
(Lan et al., 2012).
Experimental studies of the phase relations in sulphide systems have done much to inform our
understanding of the crystallization of sulphides from melts and high temperature fluids. Key binary
systems that have been studied include Fe-S, Cu-S, Ni-S and ternary systems include Fe-Cu-S, Fe-
Ni-S, Fe-Zn-S, Fe-As-S. A few quaternary systems are particularly important, notably the Fe-Zn-As-
S system. Details of the work done in these areas can be found in Vaughan and Craig (1978, 1997)
and Fleet (2006). In Figure 3 are shown phase diagrams derived using experimental data for the Fe-
Cu-S system at 700ºC and 300ºC (after Vaughan and Craig, 1997). Even at 700ºC, melts have all
5
crystallized, but notable are the extensive solid solution fields; the intermediate solid solution (iss)
which includes chalcopyrite and other phases close to CuFeS2 in composition and another field at,
and around, bornite (bn). Further cooling to 300ºC sees the shrinking of these fields, with the
associated separation by solid state diffusion that produces the exsolution textures observed using
reflected light microscopy (see Fig. 3). In the example illustrated in Figure 3, exsolved chalcopyrite
(yellow) occurs as laths in host bornite (brown); the orientation of the laths is crystallographically
controlled by the bornite host. The blue-grey phase is chalcocite formed by later alteration of the
bornite.
In another example, experimental work on the Fe-As-S system has shown that the As content of
arsenopyrite, when formed in equilibrium with pyrite and pyrrhotite, varies as a function of
temperature and, hence, can be used as a geothermometer. On the other hand, work on the Fe-Zn-S
system has demonstrated that increasing pressure reduces the iron content of sphalerite and this can
be used, under favourable circumstances, as a geobarometer.
Surface Chemistry
Sulfide surface chemistry is particularly important because of its relevance to the oxidation and
breakdown of sulfide minerals, and to industrial processing of as-mined ores using froth flotation or
leaching. Investigations have ranged from spectroscopic and imaging studies of very clean surfaces
in ultra-high vacuum (UHV) conditions providing information on structure and reactivity at atomic
resolution, to micron resolution studies of reacted surfaces and reaction products. Comprehensive
reviews are provided by Rosso and Vaughan (2006a,b).
The most studied sulfide as regards surface chemistry is pyrite; here, the major surface planes are
(100), (111), (110) and (210), with the (100) surface being considered the most stable. At the (100)
surface, a complex microtopography has been observed in UHV, defined by flat, stepped terraces,
commonly with a high step density (Figure 4a, b) (Rosso et al. 1999). Spectroscopic studies of this
surface in vacuum indicate that, upon cleavage, disulphide bonds break to form monosulphide
species (Nesbitt et al. 1998; Schaufuss et al. 1998). However, sulphur-deficient defects have also
been observed (Guevremont et al.1998). The diverse properties of sulphide surfaces are exemplified
by the electronic structure of pyrite, a 0.9 eV band gap intrinsic semiconductor; by contrast, the (100)
surface can be considered as nearly metallic, with a band gap of < 0.04 eV (Rosso et al. 1999). The
redox chemistry of pyrite in aqueous solution involves further complexities. Rimstidt and Vaughan
(2003) note that oxidation of a disulfide such as pyrite to release sulfate requires transfer of seven
electrons and, hence, up to seven elementary reaction steps. Furthermore, pyrite, like most sulfides,
6
is a semiconductor so the reactions are electrochemical in nature. It has been suggested that this
electrochemical reaction involves three distinct steps: (1) cathodic reaction, (2) electron transport,
and (3) anodic reaction, and that the cathodic reaction is the rate-determining step. It seems that the
rate of pyrite oxidation depends on the concentration of O2 or another oxidant such as Fe3+.
Other sulphides, such as monoclinic pyrrhotite (Fe7S8), also have complex surfaces. Superstructures
within the pyrrhotite family arise from vacancy ordering in layers parallel to the basal plane (Fig.
2a). In the most Fe-deficient end-member, Fe vacancies occur in every other Fe atom layer, and in
alternate rows within that layer. Furthermore, in every S atom layer, one in four S atoms relaxes into
an Fe vacancy. The (001) surface of monoclinic pyrrhotite was observed in UHV by STM to
comprise flat terraces separated by steps ~2.9Å in height (Fig 4c), which is one quarter of the unit
cell in the c direction, or the separation between two consecutive Fe or S layers (Becker et al. 1997).
Although the features appear to have an atomic arrangement suggestive of Fe vacancies, it was
ascertained that they were S atoms with an ordering induced by the Fe vacancies. Such complex
surfaces give rise to different oxidation mechanisms. For example, due to the deficiency in Fe atoms
at the pyrrhotite surface, oxidation proceeds via the formation of a sulphur rich layer, composed of
disulphide, polysulphide and elemental sulphur (Chirita and Rimstidt, 2014), while for the Fe-rich
surface layers of pyrite, ferric oxyhydroxide forms during the initial oxidation.
Given the environmental relevance, understanding the reactions at the surface of arsenopyrite during
aqueous oxidation has received much attention in the literature; as reviewed by Corkhill and
Vaughan (2009). Like the pyrite oxidation processes described above, these are complex, multi-stage
electrochemical reactions. It has been proposed that the initial oxidation of arsenopyrite surfaces
occurs rapidly, with As being oxidized more quickly than Fe. Spectroscopic studies (Schaufuss et al.
2000; Corkhill et al. 2008) have shown that oxidation of As proceeds via a series of one-electron
transfer steps, from As-1 to As5+, while S oxidation can be considered as a 7-step reaction,
transforming disulphide (S2-) to sulphate sulphur (S6+) with a number of intermediate products. These
oxidation products, which also include ferric-(oxy)hydroxides, form an oxidized surface layer
controlled by diffusion of species from the bulk (Schaufuss et al. 2000; Costa et al. 2002). Oxidative
leaching of arsenopyrite in the presence of common acid mine drainage bacteria such as
Leptospirillum ferrooxidans, greatly enhances the release of As from the surface compared with
abiotic dissolution; a combination of direct leaching, as evidenced from cell-shaped etch pits (Fig.
4d), and indirect dissolution, through cycling of Fe2+ within a thick layer of extra-cellular polymeric
substances were the determined mechanisms (Fig. 4e).
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The uptake of metal ions by sulphide surfaces has received significant interest, due to the importance
of this process in the transport and mobility of metals in the subsurface, ore formation, and the
mobility of contaminants and other pollutants. Once again, in this aspect of sulphide surface
chemistry, pyrite is the most studied phase, with other significant contributions on the readily studied
Table 2. The major types of sulphide ore deposits (modified after Cox and Singer, 1987; Craig and Vaughan, 1990) Type Major Ore Minerals* Metals Extracted Examples Ores related to mafic and ultramafic intrusions Sulphide nickel deposits po, pn, py, cpy, viol Ni, Cu, Co, PGM Sudbury, Ontario Canada Merensky reef platinum po, pn, cpy Ni, Cu, PGM Merensky Reef, RSA Ores related to felsic intrusive rocks Tin and tungsten skarns py, cass, sph, cpy, wf Sn, W Pine Creek, CA, USA Zinc-lead skarns py, sph, gn Zn, Pb Ban Ban, Australia Copper skarns py, cpy Cu, Au Carr Fork, Utah, USA Porphyry copper/ py, cpy, bn, mbd Cu, Mo, Au Bingham Canyon, molybdenum Utah; USA Climax, CO, USA Polymetallic veins py, cpy, gn, sph, ttd Cu, Pb, Zn, Ag Camsell River, NWT, Canada Ores related to marine mafic extrusive rocks Cyprus-type massive py, cpy Cu Cyprus sulphides Besshi-type massive py, cpy, sph, gn Cu, Pb, Zn Japan sulphides Ores related to subaerial felsic to mafic extrusive rocks Creede-type epithermal py, sph, gn, cpy, ttd Cu, Pb, Zn, Ag, Au Creede, CO, USA veins asp Almaden-type mercury py, cinn Hg Almaden, Spain Ores related to marine felsic to mafic extrusive rocks Kuroko-type py, cpy, gn, sph, ttd, asp Cu, Pb, Zn, Ag, Au Japan Ores in clastic sedimentary rocks Quartz pebble U-gold py, uran, gold Au, U Witwatersrand, RSA Sandstone-hosted py, sph, gn Pb, Zn, Cd Laisvall, Sweden lead-zinc Sedimentary exhalative py, sph, gn, cpy Cu, Pb, Zn, Au, Ag Sullivan, BC, lead-zinc asp, ttd, po Canada Tynagh, Eire Ores in carbonate rocks Mississippi Valley type py, gn, sph Zn, Pb, Cd, Ga, Ge Missouri, USA
*Abbreviations used are as follows: po-pyrrhotite, pn-pentlandite , py-pyrite, cpy-chalcopyrite,viol-violarite, cass-cassiterite, sph-sphalerite, wf-wolframite, gn-galena, bn-bornite, mbd-molybdenite, ttd-tetrahedrite, asp-arsenopyrite, cinn-cinnabar, uran-uraninite.
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Figure 1 Crystal structures of the major sulphides: (a) galena (PbS), (b) sphalerite (ZnS), (c) wurtzite (ZnS);
(d) niccolite (NiAs); (e) linkage of metal–sulphur octahedra along the c-axis direction in pyrite (FeS2),
marcasite (FeS2), loellingite (FeAs2) and arsenopyrite (FeAsS); (f) covellite (CuS) and (g) cube cluster of
tetrahedrally coordinated metals in the pentlandite ((Ni,Fe)9S8) structure. (Adapted from Craig and Vaughan
1990). Atom colours: light blue – Pb; purple – Zn; yellow – S; red – Fe; orange – As; dark red – Ni; dark blue
– Cu; in arsenopyrite, yellow/grey spheres are either S or As.
17
Figure 2. Parent and derivative crystal structures in the sulphide minerals: (a) the structure of niccolite (NiAs), parent of
derivatives troilite (FeS) and monoclinic pyrrhotite (Fe7S8). Also shown are atoms in the basal plane of troilite with
arrows indicating atom movements when distortion occurs; the ordered omission of iron atoms in monoclinic pyrrhotite
is also shown, with vacancies represented by squares (only the iron-atom layers are shown); (b) the sphalerite (ZnS),
chalcopyrite (CuFeS2) and stannite (Cu2FeSnS4) structures; and (c) the sphalerite and chalcopyrite unit cells with an
octahedral site outlined which is empty in chalcopyrite, but within which are located the additional metal atoms in the
‘stuffed derivatives’ talnakhite (Cu9Fe8S16), mooihoekite (Cu9Fe9S16), and haycockite (Cu4Fe5S8). The ordered
arrangement of additional metals in these octahedral sites, and the dimensions in terms of the parent sphalerite cell are
also shown (Adapted from Craig and Vaughan, 1990). Atom colours: purple – Zn; yellow – S; red – Fe; orange – As;
dark red – Ni; dark blue – Cu; blue/grey – Sn.
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Figure 3. Phase relations in the Cu-Fe-S system at 700°C and 300°C. Apart from the pure Cu, Fe, and S end-members,
at 700ºC, pyrite (py) is a stable phase and pyrrhotite (po) has a small area of solid soloution coloured blue. There are two
large fields of solid solution (also coloured blue), one centred around bornite (bn) and the other the so-called intermediate
solid solution (iss) which centres around chalcopyrite and related minerals. On cooling to 300ºC all three solid solution
fields decrease in area. New phases now stable are covellite (cv), chalcocite (cc) and idaite (id). Chalcopyrite (ccp)
exsolves from the iss as it shrinks on cooling. Also shown is a reflected light microscopy image (Width of Field =
900たm) showing crystallographically oriented chalcopyrite laths (yellow) now hosted by bornite (brown). The blue-grey
areas are chalcocite alteration of the bornite. The sulphides are in a silicate mineral host and the sample is
typical of a porphyry copper deposit (see Table 2).
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Figure 4. Images of sulphide surface structures, showing (a) STM observation of large stepped (100) pyrite
(FeS2) terraces in the <10> direction, (b) which were commonly present at a high step density (after Rosso et
al. 1999); (c) STM images of monoclinic pyrrhotite (Fe7S8), showing ~2.9 Å step height and triangles of
groups of three S atoms in alterate directions of orientation on neighbouring layers across the 2.9 Å step (after
Becker et al. 1997); (d) bacterial leach pits on the surface of arsenopyrite (FeAsS) reacted in the presence of L.
ferrooxidans, which were found below a layer of (e) extracellular polymeric substance, hypothesised to act as
a dual direct and indirect oxidation mechanism for enhanced arsenic release (after Corkhill et al. 2008).
20
Figure 5. The pathways leading directly to the formation of pyrite or other iron sulphides (mackinawite and
greigite) in the sedimentary environment (information taken from Berner, 1984; Lennie & Vaughan, 1996;